Coatings for steel

ABSTRACT

A FERROUS OXALATE COATING ON FERROUS METAL RESULTS FROM CONTACTING THE METAL WITH AN AQUEOUS SOLUTION OF FERROUS OXALATE, FERRIC ION, FLUORIDE ION, OXALIC ACID AT A PH OF 0-2.5 AND A TEMPERATURE OF 130* TO ABOUT 190*F. A MIXTURE FOR PREPARING AND REPLENISHING THE BATH CONSISTS OF OXALIC ACID, FERRIC SULFATE AND ALKALI METAL BIFLUORIDES.

United States Patent O 3,649,371 COATINGS FOR STEEL Singkata Tongyai, Warrington, Pa., assignor to Pennwalt Corporation, Philadelphia, Pa. N Drawing. Filed Aug. 8, 1969, Ser. No. 848,689 Int. Cl. C23f 7/20 US. Cl. his-6.14 A 2 Claims ABSTRACT OF THE DISCLOSURE A ferrous oxalate coating on ferrous metal results from contacting the metal with an aqueous solution of ferrous oxalate, ferric ion, fluoride ion, oxalic acid at a pH of 02.5 and a temperature of 130 to about 190 F. A mixture for preparing and replenishing the bath consists of oxalic acid, ferric sulfate and alkali metal bifluorides.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to the art of coating ferrous metal and is particularly concerned with a method and a composition of matter useful for coating metals with an oxalate coating. The invention also relates to the preparation and replenishment of coating baths for ferrous metals.

Description of the prior art Virtually all the processes for chemically coating ferrous metals, i.e. mild steel and stainless steel, employ aqueous solutions of oxalic acid. The expression stainless steel is used in its generally accepted meaning. A detailed list of steels which fall Within the term stainless steels can be found in recent editions of the Metals Handbook, published by the American Society for Metals. Stainless steels referred to as the 200, 300, and 400 series contain elements such as chromium and nickel either with or without cobalt and molybdenum. These ferrous metals when coated in an oxalate process receive an oxalate coating on their surface which acts as a lubricant and as a lubricant carrier during metal deformation operations.

Many of the commercial aqueous oxalic acid coating solutions contain a sulphur compound containing oxygen which liberates sulphur, hydrogen sulfide and/or sulphur dioxide under the acidic conditions of the coating solution. These additives are called accelerating agents and are normally chosen from the class consisting of sulfides, sulfites, bisulfites, hydrosulfites, thiosulfates, and thionates. Usually in the form of an alkali metal salt. These accelerating agents have many objectionable features:

(1) They produce strong and unpleasant smelling byproducts which are toxic.

(2) They are continuously consumed and must be continuously replaced. Since exact control of the replenishing process is diflicult to achieve the concentration of these agents in the bath varies during the life of the bath.

(3) They cause control problems due to the presence of sulphur containing sludge in the bath which constantly builds up as the bath is used.

(4) They add to the total cost of the bath.

(5) They are temperature sensitive and limit the bath operating temperature or necessitate implementing an oil layer on the bath as per US. Pat. 2,953,487.

There are also oxalate coating baths which do not use sulphur compounds of the kind mentioned. They utilize, in many cases, an oxidizing compound e.g. chlorates, nitrites and organic nitro compounds. These are difllcult to use because, if not carefully controlled, they may oxidize either the oxalic acid in the bath or the oxalate coating on metals.

SUMMARY OF THE INVENTION This invention is concerned with a ferrous metal coating composition comprising an aqueous solution contaming:

(a) Suflicient ferrous oxalate to saturate the solution; (b) A minimum of two percent oxalic acid;

(0) A minimum of 0.2 percent ferric ion;

(d) A minimum of 2.0 percent fluoride ion;

and having a pH of 0 to about 2.0.

Optionally, the composition may include about two grams per liter of sulfate ion.

A process for using this composition involves contacting it with ferrous metal while maintaining the solution at a temperature of to about 200 F.

A composition for preparing and replenishing the bath consists essentially of oxalic acid, ferric sulfate and a source of fluoride ion. The fluoride ion is derived from the group consisting of alkali metal fluorides, alkali metal bifluorides, ammonium fluoride, ammonium bifluoritle, and hydrogen fluoride.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The coating solutions of this invention are prepared by adding the following chemicals to water:

(1) Suflicient ferrous oxalate to saturate the solution at the operating temperature and pH. Initially, lesser amounts may be employed because additional ferrous oxalate is generated during the coating operation. The ferrous oxalate may be derived from ferrous salts and oxalate ion added to the bath.

(2) A minimum of two percent by weight of oxalic acid, i.e. about 20 grams per liter. The range is from about 20 grams per liter to about 400 grams per liter. A coating is formed with less oxalic acid present but the resulting coating does not adhere as tightly to the metal as it does when larger amounts of oxalic acid is present.

Oxalic acid may also be obtained in the bath by indirect means, i.e. from hydrogen ion and oxalate ion. Sources of oxalate ion can be for example; ferric or ferrous oxalate. Examples of hydrogen ion sources are sulfuric acid, alkali metal bisulfates and alkali metal bifluorides.

Normally, mineral acids are added to the bath in order to adjust the pH. However, if desired, oxalic acid may be used in place of the mineral acids. Preferably, about 14.5 percent by Weight of oxalic acid dihydrate is added when other acids are excluded from the bath.

(3) Ferric ion, from about 0.2 gram per liter to about 10 grams per liter. Preferably, the ferric ion is about 5 grams per liter and is added as the sulfate. Other sources of ferric ion may also be used, e.g. hydrated ferric oxide, ferric hydroxide, ferric fluoride or a ferric fluoride complex. The amount of ferric ion in the solution can be high, e.g. at the saturation level for the particular ferric salt, however, some free oxalic acid must also be present in the bath in order to obtain more adherent coatings.

(4) Fluoride ion from about 2 percent to about 20 percent by weight, i.e. from about 20' grams per liter to about 200 grams per liter. More preferred is 2.15 percent to about 6 percent fluoride salt. Less than 2 percent fluoride ion may be used while coating certain steels, e.g. mild steel and 400 series steel, but higher operating temperatures and longer operating times are required. About 2.8 percent fluoride ion is recommended for 300* series stainless steel.

Fluoride ion used herein may be derived from fluoride complexes of iron hydrogen fluoride or any fluoride or bifiuoride salt. Hydrogen fluoride, ammonium bifluoride and the alkali metal bifluorides'are preferred.

The pH of the coating solution is within the range of to 2.5. More preferred is the pH range of 0.2 to 1.2. When the pH is less than 0.2, extra care must be practiced in using the bath to avoid over etching the metal to be coated. Bath pHs higher than 2.5 result in low quality coatings, because the metal surfaces are not sufficiently reactive under these conditions.

The pH of the bath may be adjusted using mineral acids, e.g. sulfuric, hydrofluoric, etc. Oxidizing acids, such as nitric acid may be used but are generally not preferred because they may attack the oxalate coating. The bath pH may may also be adjusted using organic acids. Oxalic acid is especially suitable for this purpose because it serves both as a source of hydrogen ion for pH purposes and oxalate ion for coating purposes. Both oxalic and sulfuric acid are preferred for pH adjustment.

The operating temperature of the bath during coating is from 130 F. to about 200 F. More preferred is the operating temperature range of from 150 F. to about 185 F. Below the lower temperature range mentioned, the coating reaction is too slow to be commercially significant. Above the higher temperature range too much etching of the metal occurs. Exceeding either end of the temperature range may affect the adhesion of the coating.

The stainless steels and mild steels which are coated by the coating solutions and processes of this invention vary in their composition. This factor causes them to behave differently in the coating solution. This is exhibited in the amount of time and the temperature a given steel is exposed to a bath in order to obtain an optimum coating. For example, good coatings on 400 series stainless steels may be obtained in four to seven minutes at 150- 155 E; series 316 and 304 stainless steel were observed to achieve good coatings at 160165 F. in seven minutes; and series 347 and 321 required to minutes at 165175 F.

The composition of this invention may optionally include sulfate ion. The presence of sulfate ion has been found to produce coatings on the metal surface having higher adherence than is obtainable in its absence and the preferred compositions of this invention include sulfate ion, usually obtained by adding ferric sulfate. The amount of sulfate ion can be small, e.g. five grams per liter of ferrous sulfate provides enough sulfate ion to yield good coatings. Ferric ion is usually added as a sulfate, and therefore, the sulfate ion is present in amounts equivalent to the added ferric ion.

The time required to form a coating on the metal surface varies from five to twenty-five minutes when the bath is at 155 F. to about 185 F. These conditions will provide a good coating on all mild and stainless steels. Longer or shorter coating times can be effected by chang ing the temperature or concentration of ingredients in the bath. Longer or shorter times will also be indicated depending upon coating thickness.

Etching of the metal surface to be coated prior to its exposure to the coating bath is not required but is preferred. The main difference observable is that the resultant coatings are more even when the metals are etched prior to coating. This is believed to result because etching removes a mixture of metal oxides formed on the surface. These oxides are formed by oxidation of the mixture of metals present in stainless steel. These oxides vary in their rate of removal when the metal is in contact with the bath and thereby cause an uneven coating.

The-bath can be prepared or replenished by means of a packaged mixture containing the following ingredients in the indicated amounts.

Percent Ingredient: by weight Ferric sulfate (anhydrous) -50 Oxalic acid -75 Sodium bifiuoride 0-15 This mixture is easily formulated and because its ingredients are not generally considered oxidizing or reducing agent-s, they are stable during storage. Other sources of ferric and fluoride ion may also be utilized to prepare a replenishing package. Additional fluoride ion than is indicated in the above formulation may be desired, especially if the bath is being prepared for the first time. Bulk sodium bifluoride may be added to raise the fluoride level of the bath. When the above package is used to replenish the bath, additional fluoride ion is not usually required. The coating ingredients of the bath are, of course, consumed or depleted during the treatment of a succession of articles. The depletion rate of the components in the baths of this invention is in part dependent upon the manner in which the bath is used. If the bath is operated on a continuous basis, the coating producing ingredients are consumed on a relatively uniform basis. Accordingly, replenishment of the bath is most readily and conveniently accomplished by adding the mixture as shown in Example 7. This will restore the three ingredients to the desired level. Under some circumstances, as for example; when the bath temperature is not controlled, one or more ingredients may be excessively consumed. If this happens, the individual ingredient may be replenished by supplementing the replenishing mixture.

If desired, the coating baths of this invention may be replenished in the usual manner familiar in the art which consists of adding the individual coating constituents as required.

Replenishment of the coating chemicals is effected in accordance with the needs as determined by pointage titration which technique is well known in the coating industry. It is obvious from the foregoing that the use of the admixtures prepared in the ratio indicated herein makes it possible to prepare and maintain a bath by weighing or measuring only one admixture which, of course, reduces the possibility of error in calculation which is inherent in the prior practice of separately weighing and adding different ingredients to the bath. This advantage is also present in situations where titrations indicate that one or another of the ingredients in the bath has been excessively consumed. In this situation, it is merely necessary to calculate how much more of the depleted ingredient must be added over that contained in the prepackaged composition. Illustrative examples of coating solutions falling under the purview of this invention are listed below, but are in no way intended to limit its scope.

EXAMPLE 1 A bath was prepared by dissolving 5 grams per liter ferrous sulfate (hydrated) and 86.7 grams per liter ferric oxalate (Fe (C O 6H O) in water. Sulfuric acid was added to lower the pH of bath without fluoride to 0.7 and then grams per liter sodium bifluoride was added. This bath coated 304 type stainless steel in 5 to 7 minutes at F. to F.

EXAMPLE 2 A bath as described in Example 1 was prepared. About 110 grams per liter of oxalic acid was also added to the bath. Type 304 stainless steel was coated under the same conditions of Example lthis coating was improved in appearance and adhesion over that of Example 1.

EXAMPLE 3 A bath was prepared containing 95 grams per liter ferric oxalate, 8 grams per liter ferrous oxalate, 108 grams per liter oxalic acid (H C O -2H O) and 77.5 grams per liter sodium bifluoride. With this bath 304 stainless steel was coated at 165 F. to F. for seven minutes. A coating weight of 1800 mg./sq. ft. was obtained.

EXAMPLE 4 A bath was prepared containing 64.8 grams per liter anhydrous ferric sulfate, 2.1 grams per liter ferrous sulfate, 132.8 grams per liter oxalic acid (as the dihydrate) and 52.5 grams per liter of sodium bifiuoride. A coating of 2600 to 2800 mg./sq. ft. and having good adhesion was produced in seven minutes at a bath temperature of 165 F. to 175 F. on type 304 stainless steel.

EXAMPLE A bath containing 3.5 grams per liter ferrous sulfate, 100 grams per liter ferric sulfate (technical grade with Fe 300 grams per liter oxalic acid and grams per liter sodium bifluoride and 5.5 grams per liter concentrated sulfuric acid produced 3100 mg. to 4300 mg./ sq. ft. of adherent coating at 155 F. to 162 F. in seven minutes on 304 type stainless steel.

EXAMPLE 6 A bath was prepared containing grams per liter anhydrous ferric sulfate, 220 grams per liter oxalic acid,

90 grams per liter sodium bifluoride and 2 grams per liter ferrous sulfate. This bath was heated to 180 F. for

at least one hour to dissolve all of the ferric sulfate and A bath was prepared by mixing 39.4 grams of ferric sulfate, 0.6 gram of ferrous sulfate, 85 grams of oxalic acid and 75 grams of sodium bifluoride and enough Water to make 800 milliliters of solution. The ingredients were prepackaged in this ratio and added to water as indicated. The bath was stirred and heated at 180 F. until the salts dissolved. The bath was used on 300 stainless steel panels until about 90 percent of its original concentration was consumed as determined by analysis for iron and oxalate content. The bath was then replenished using a mixture composed of the following:

Ingredient: Percent by Weight Ferric sulfate 46.0 Oxalic acid 45.0

Sodium bifiuoride The compositions and process of the invention are useful for preparing adherent oxalate coatings on stainless steel without using sulfur accelerators. Accordingly they avoid the problems associated with coating solutions containing sulfur accelerators. Sulfur accelerators is understood to mean reducing compounds such as sulfites and thiosulfates and does not include sulfates.

Thus the composition and process of this invention are easy to control because they do not require an accelerator ingredient and do not produce sulfur-containing sludge.

What is claimed is:

1. In a process for treating the surface of stainless steel to produce an oxalate coating thereon, the improvement comprising contacting the stainless steel surface with an aqueous acidic solution consisting essentially of oxalic acid, ferric ion, fluoride ion, sulfate ion and saturated with ferrous ion for a time interval suflicient to form a substantial coating and at a reactive temperature within the range of from F. to 200 F. and wherein the solution has a pH range from 0.2 to 2.5.

2. The process in accord with claim 1 in which the aqueous acidic solution consists essentially of (a) Oxalic acid within the range of 20 to 400 grams per liter;

(b) Fluoride ion within the range of 20 to 200 grams per liter;

(c) Ferric ion within the range of 0.2 to 10 grams per liter;

(d) Sulfate ion within the range of 2 to 5 grams per liter; and

(e) Ferrous ion at saturation level.

References Cited UNITED STATES PATENTS 1,895,568 1/1933 Curtin et a1 148-614 2,273,234 2/ 1942 Tanner 148-6.l4 2,550,660 5/1951 Amundsen et al. 148-614 2,577,887 12/1951 Gibson 148-614 2,785,097 3/1957 Goodspeed et al. 148-614 2,800,421 7/1957 Goodspeed et al. 148-614 2,846,341 8/1958 Johns et al. 148-614 2,935,431 5/1960 Shaw et al. 1486.14

RALPH S. KENDALL, Primary Examiner US. Cl. X.R. 148-624 

